The field of application of the invention relates in particular, but not exclusively, to telephone speech circuits incorporated to telephone subscriber bets. Reference will be made in this description to that field of application for simplicity of illustration.
Specifically in that field of application, there exists a demand for RC filters operating at a low or very low frequency. Further, frequently associated with this demand is the need to achieve very high time constants, e.g. on the order of a fraction of a second. Exemplary of this are those instances where a DC reference voltage requires to be separated from the telephone signal being received in said speech circuits.
As is known, a very high time constant requires that very large capacitors and/or resistors be also used.
This is not much of a problem where discrete component circuits are involved, because capacitors on the order of hundreds of microfarads and resistors on the order of tens of Megaohms are readily available commercially.
However, where such components are to be implemented in an integrated circuit, serious problems arise in terms of the silicon area to be reserved for the RC circuit.
The prior art has been proposing the use of discrete components connected externally of the integrated circuit. This is the approach that has been followed heretofore for telephone speech circuits as well.
But this prior approach tends to make the manufacturing process of each silicon chip, and hence each circuit, more expensive. Additionally, prior solutions have proved low in accuracy.
The underlying technical problem of this invention is to provide an RC filter which has such structural and operational features as to suit integrated circuits requiring very high time constants and overcome the limitations which beset prior art embodiments.